Ceramic body of zirconium dioxide (ZrO2) and method for its preparation

ABSTRACT

A ceramic body of zirconium dioxide containing, if desired, aluminum oxide,nd partially stabilized with yttrium oxide and/or one or more rare earth oxides (e.g., cerium dioxide) and/or magnesium oxide and/or calcium oxide is partially stabilized with 0.5 to 5 mole-% of yttrium oxide and/or 5 to 12 mole-% of magnesium oxide and/or calcium oxide and/or cerium dioxide or one or more rare earth oxides, is 30 to 100% in the tetragonal lattice modification and has in the surface region a content of yttrium oxide, cerium dioxide, magnesium oxide, calcium oxide or rare earth oxide that is 1 to 20 mole-% higher than the average content, such that the body is coated with a thin, PSZ-like layer in a more highly stabilized tetragonal or with a layer that is predominantly in the cubic lattice form. For preparation, the surface of an already sintered or only presolidified compact of partially stabilized zirconium oxide is brought into intimate contact with yttrium oxide, cerium dioxide, magnesium oxide, calcium oxide and/or another rare earth powder or a zirconium dioxide powder containing at least 12 mole-% of yttrium oxide and/or other stabilizer oxides, and then annealed at 1000° to 1600° C. until a more highly stabilized tetragonal or predominantly cubic surface layer of 0.1 to 200 micrometers thickness and 2 to 20 mole-% higher content of yttrium oxide, cerium dioxide, magnesium oxide, calcium oxide or rare earth oxide has formed.

Finely granular zirconium dioxide bodies partially stabilized withyttrium oxide, cerium dioxide and/or other rare earth oxides, andcoarsely granular zirconium dioxide bodies partially stabilized withmagnesium oxide or calcium oxide pertain to the polycrystalline ceramicswhich have the highest strengths and resistance to fracture which havebeen measured up to now. The chief reason for this lies in thetension-induced transformation of the tetragonal lattice modification tothe monoclinic room-temperature modification. For example, bodiescontaining yttrium oxide are sintered, hot-pressed or hot-isostaticallypressed ("hipped") usually with an yttrium oxide content between 1 and 6mole-%, either in the tetragonal monophasic field or in thecubic/tetragonal two-phase region, at temperatures between 1400° and1550° C. Their structure then consists of a fine-grained (0.1-1.0micrometer), tetragonal content (up to 100%) and somewhat coarser, cubicgrains (1-10 micrometers) (3.5-6.0 mole-% for high yttrium oxidecontents). To increase the hardness and modulus of elasticity the bodiescan contain aluminum oxide in larger amounts.

Zirconium oxide bodies containing magnesium oxide or calcium oxide areusually sintered in the cubic monophasic region at temperatures between1690° and 1800° C.; they are therefore more coarse-grained (50 to 70micrometers).

The decisive disadvantage of these super-strong ceramic bodies,especially those containing yttrium oxide, is that they lose theirstrength drastically after relatively long heat treatment attemperatures between 200° and 550° in air; this loss of strength isgreatly accelerated with increasing atmospheric humidity or high steampressure (O. T. Masaki, K. Kobayashi, Proc. Ann. Meeting Jap. Ceram.Soc. 1981). Even in warm, aqueous solutions, degradation of the bodiescan occur. The reason for this is not yet understood. It is assumed,however, that the mechanical tensions of the tetragonal bodies areremoved by tension crack corrosion at the grain boundaries, and thus atransformation to the monoclinic form occurs, or that otherdiffusion-controlled mechanisms cause martensite nuclei to form at thesurface and thus initiate the transformation that ultimately results inthe destruction of the bodies.

This decisive disadvantage makes the new class of the so-called TZPceramics (TZP: Tetragonal Zirconia Polycrystals. A bibliography on TZPceramics is contained in the book, "Science and Technology of ZirconiaII", Advances in Ceramics, Vol. 11, 1984) suitable for use in air onlyfor application temperatures up to about 200° C., although such ceramicswould offer substantial advantages for use in internal combustionengines. This phenomenon would also be disadvantageous for use as abioceramic (hip joint replacement).

Conventional zirconium dioxide ceramics partially stabilized withmagnesium oxide (Mg-PSZ), when exposed to heat for long periods atsomewhat higher temperatures (700° to 1000° C., have a similardisadvantage. On account of the rapid diffusion or vaporization rate ofmagnesium oxide, surface degradation occurs, especially in a slightlyreducing atmosphere.

Surprisingly, it has now been found--and the invention is based onit--that in sintered specimens which have been heat treated in a milieurich in yttrium oxide, cerium oxide, magnesium oxide or calcium oxide,e.g., a powder bed of yttrium oxide or magnesium oxide, this degradationprocess does not occur, or occurs to a lesser degree.

The invention is therefore based on the problem of reducing oreliminating the above-described loss of strength or surface degradationin ceramic bodies of zirconium dioxide.

The problem is solved by a ceramic body partially stabilized withyttrium oxide and/or cerium oxide and/or one or more rare earth oxidesand/or magnesium oxide and possibly containing aluminum oxide, which ischaracterized by being partially stabilized with 0.5 to 5 mole-% ofyttrium oxide and/or 2 to 12 mole-% of magnesium oxide and/or calciumoxide and one or more rare earth oxides (e.g., cerium oxide), is 30 to100% in the tetragonal lattice modification, and has in the surfaceregion a content of yttrium oxide or rare earth oxide and/or magnesiumoxide and/or calcium oxide, such that the body is covered by a thinlayer that is mostly in the cubic lattice form or in a more highlystabilized tetragonal lattice form. It is obvious that a partiallystabilized cubic layer can be transformed by tempering (peak aging) attemperatures commonly used in PSZ (1100°-1420° C.) to a PSZ-like layer(i.e., cubic grains having tetragonal segregations).

The term, "thin surface layer," in the sense of the invention, is to beunderstood to mean a layer with a thickness of 0.1 to 200 micrometers,preferably 0.3 to 30 micrometers. The body on the basis of zirconiumdioxide in accordance with the invention is prepared by firing it in amilieu which is rich in yttrium oxide, cerium oxide, magnesium oxide,calcium oxide and/or rare earth oxides. The invention is explainedhereinbelow on the basis of the use of yttrium oxide, cerium oxide,magnesium oxide and calcium oxide. It is to be understood, however, thatit applies likewise to other rare earth oxides. This surfacestabilization or annealing is likewise advantageous for conventionalzirconia ceramics partially stabilized with magnesium oxide or calciumoxide.

To prepare the ceramic body in accordance with the invention, it ispossible to set out either from the finished sintered or hipped ceramicor from a green body presolidified at relatively low temperature (e.g.,room temperature). The ceramic or the green body is now provided with asurface of yttrium oxide, cerium oxide, magnesium oxide, calcium oxide,etc., either in the form of a pressed-on layer of powder or of a slipcontaining yttrium oxide or magnesium oxide, which can be sprayed on,for example, or applied in the form of a bath for impregnating thesurface. The bodies thus treated are then fired or sintered attemperatures between 1000° and 1600° C., the length of the treatmentbeing able to be between about 10 minutes and about 100 hours. Thedesired surface stabilization is also achieved to special advantage byfiring or sintering the ceramic or green body in a powder bed of yttriumoxide and/or cerium oxide and/or magnesium oxide and/or calcium oxide.Those conditions are preferred in which the desired diffusion isachieved in the shortest possible time, while at the same time achievinga PSZ-like layer.

For the preparation of the ceramic body itself, the body can beperformed either by mixing the oxides, or by wet chemical methods suchas sol gel, coprecipitation, spray reaction of aqueous solutions, orfrom fine, homogeneous powders obtained by fusion and prealloyed withyttrium oxide, cerium oxide, magnesium oxide and/or calcium oxide, andthen sintering or hipping, or sintered and then hipped, at temperaturesgenerally between 1350° and 1550° C. The finished ceramic is then, asmentioned above, coated with yttrium oxide, cerium oxide, magnesiumoxide, calcium oxide, etc., or fired in a corresponding powder bed,until the surface layer enriched with yttrium, cerium, magnesium, orcalcium oxide etc. is formed.

When a stabilizer-rich coating is applied to a green body, the body iscommonly preformed at a low pressure, say of about 100 MPa, and thenpressed again at higher pressure, e.g., 200 to 650 MPa. In most cases,however, the preferred method is the sintering of the pressed body orthe firing of a finish-sintered and processed body in a powder bedcontaining magnesium oxide or yttrium oxide and/or cerium oxide.

The ceramic bodies of the invention, in comparison to specimens preparedunder otherwise equal conditions but without the above-described surfacetreatment, in a treatment for accelerated aging, consisting of fourhours of firing at temperatures between 250° C. and 400° C. at steampressures of 4 to 15 bar, show scarcely any effect.

In X-ray examination, in the case of the ceramic bodies of theinvention, only the cubic and tetragonal reflections of the bodysubjected to the accelerated aging are detectable after this treatment,while the specimens used for comparison show strong monoclinicreflections which are an indication of incipient degradation. The bestresults were obtained when the thin surface layer was produced by firingthe ready-sintered samples in magnesium oxide, yttrium oxide, ceriumoxide or calcium oxide powder, or by treatment with yttrium oxide powderor a zirconium powder containing at least 12 mole-% of yttrium oxide,the surface layer being pressed onto the zirconium oxide compactsstabilized by a small addition (0.5 to 5, preferably 2 to 4 mole-%) ofyttrium oxide, or being applied as an aqueous suspension of powder andsintered. But, no matter how the surface layer is produced, an importantcondition for the achievement of the protective action of thestabilizer-containing coating is very close contact with the surface ofthe zirconium oxide specimen to be heated or sintered.

The thin, generally 0.5 to 30 micrometers deep, stabilizer-richzirconium oxide surface layer which is formed by the treatment of theinvention, appears to constitute a protection against long-term thermaldisintegration. This layer can also contain aluminum oxide for finingthe grain. Presumably other rare earth oxides produce a similarlypositive effect, as previously mentioned. On account of the extremelyslow diffusion of yttrium oxide into zirconium oxide at temperaturesbelow 1000° C., this layer represents primarily a thermally stableprotection for TZP ceramics, but also for conventional zirconium oxidepartially stabilized with magnesium or calcium (Mg-, Ca-PSZ).

The following examples further explain the invention.

EXAMPLE 1

Samples of reaction-sprayed powders (EDS powders: EvaporationDecomposition of Solutions, Am. Ceram. Soc. Bull 50 (1977) 1023) whichcontained 2 mole-% of yttrium oxide and 1.5 vol.-% of aluminum oxide,and had been ground for 4 hours in water in an attrition mill withalumina balls containing silica and spray dried, were isostaticallypressed at 630 MPa and sintered in air for 2 hours at 1450° C. The X-rayreflections thereafter indicated a predominantly tetragonal structure(grain size approx. 0.4 micrometers). Flexural test samples indicate, inthe polished surface state, a strength of 920 MPa (type I) and, after 36h of firing at 1350° C. in an yttria powder bed, a strength of 810 MPa(type II). After all of the samples were cooked in the autoclave at 400°C. for 4 hours at 4 bar steam pressure, the strength of type I was only420 MPa, while type II showed a strength of 740 MPa.

EXAMPLE 2

Samples of a powder which was prepared and treated as in Example 1, butcontained only 2 mole-% of yttrium oxide by volume, were formed as inExample 1. An aqueous suspension of yttria powder was applied to thecylindrical compacts and some of it penetrated into the surface pores;then the coated compacts (type I) were sintered at 1500° C. for 2 hours,and then subjected to the autoclaving described in Example 1, togetherwith identical samples with no coating (type II). After this treatment,type I showed only tetragonal and cubic X-ray reflections, but type IIshowed tetragonal and large monoclinic X-ray reflections which indicatesthe thermal degradation of the uncoated samples.

EXAMPLE 3

Samples from the powder of Example 1 were isostatically pressed at apressure of 100 MPa, and then sprayed with a suspension of 12 mole-%zirconia powder containing 12 mole-% of yttrium oxide (coating thicknessapprox. 40 to 200 micrometers), then pressed again isostatically at 630MPa, and sintered as in Example 1. After the autoclaving treatment (asin Example 1), no thermal degradation of the surface could be detected.

EXAMPLE 4

Samples in accordance with Example 2 were coated with the samesuspension, but this time with the addition of 20% alumina by volume,and otherwise treated as in Example 1. Here, again, no degradation couldbe detected after the heat treatment in the autoclave.

EXAMPLE 5

50 volume-percent of alumina powder (Pechinee Ugine Kuhlman, A6) [wasadded] to the powder from Example 1 and ground in the attrition mill asin Example 1. Isostatically pressed cylinders (approx. 1×1 cm diameter)were sintered at 1500° C., some with (type I) and some without (type II)a slip of 50 wt.-% of yttrium oxide and 50 wt.-% of cerium oxide. Thentype I contained on the polished surface only tetragonal zirconium oxideplus aluminum oxide (measured by X-ray analysis), while type IIadditionally contained cubic forms. After autoclaving as in Example 8,with only 8 bar of steam pressure, the surface of type I had a highcontent of monoclinic zirconium oxide, while type II showed nomeasurable change.

EXAMPLE 6

A coprecipitated zirconia powder containing 2.2 mole-% of yttrium oxidewas pressed isostatically at 620 MPa; the samples were then sintered inair for 2 hours at 1500° C. The bodies thus prepared containedexclusively tetragonal grains of an average size of 0.4 micrometers(material type A). A similarly made commercial material with 3 mole-% ofyttrium oxide contained approximate 80% of tetragonal grains(approximately 0.4 micrometers) and approximately 20% cubic grains(about 5 micrometers) (material type B).

Material types A and B were subjected to an autoclave test with a steampressure of 5 bar at 250° C. for 2 hours, and both types degradegreatly, i.e., show mostly monoclinic reflections at the surface; type Awas even completely decomposed.

Types A and B were then fired each for 2 hours in powder beds of yttriumoxide, cerium oxide, titanium oxide, magnesium oxide and calcium oxide,at different temperatures. The heat treatment temperatures and theresults of the autoclave test that followed are listed in Table 1. Fromthis it appears that, with the exception of titanium oxide, all theother oxides have a positive effect, especially at higher temperatures.A firing in a magnesium oxide powder bed is effective even at relativelylow temperatures (1120° C.).

Types A and B, in the form of unsintered compacts, were sintered for 2 hat 1500° C. in powder beds of yttrium oxide, cerium oxide, calcium oxideand magnesium oxide (in air). The above-described autoclave test againshowed no surface degradation.

                  TABLE 1    ______________________________________    Sintering in a powder bed, followed by autoclave test for 2    hours, 5 bar steam pressure, 250° C.    Powder bed           Yttrium  Cerium   Titanium                                    Magnesium                                            Calcium    Sintering           oxide    oxide    oxide  oxide   oxide    tempera-           Type of Material    ture °C.           A      B     A    B   A    B   A     B   A    B    ______________________________________    1120   -      -     -    -   -    -   -     -   -    o    1220   -      o     o    o   -    -   -     o   +    +    1320   o      +     o    +   -    -   +     +   +    +    1420   +      +     +    +   -    o   +     +   +    +    ______________________________________

EXAMPLE 7

A conventional zirconia partially stabilized with magnesia (Mg-PSZ),containing 3.3 wt.-% of magnesium oxide, was subjected to a solutionanneal in air at 1700° C. for 2 hours, followed by rapid cooling to roomtemperature, and was then subjected to two hours of sintering at 1420°C. in yttria powder. While the monoclinic content at the surface in theuntreated (as-received) sample increased, after 100 hours in a slightlyreducing atmosphere at 920° C., from originally 15% to 32%, themonoclinic content in the sample sintered in yttria was below themeasurable range, i.e., less than 4%.

We claim:
 1. In a ceramic body of zirconium dioxide or zirconium dioxidecontaining aluminum oxide, the improvement comprising said ceramicbodybeing partially stabilized with 0.5 to 5 mole-% of yttrium oxide, 5to 15 mole-% magnesium oxide, calcium oxide, 5 to 15 mole-% ceriumoxide, 5 to 15 mole-% of one or more rare earth dioxides or 5 to 15mole-% of a combination thereof; being 30 to 100% in the tetragonallattice modification; and having, in the surface region, a content ofyttrium oxide, cerium dioxide, magnesium oxide, calcium oxide or rareearth oxide that is 2 to 20 mole-% higher than the average content insaid ceramic body, such that the body is covered with a thin layer of amore highly stabilized tetragonal lattice form or one predominantly inthe cubic lattice form.
 2. The ceramic body of claim 1partiallystabilized with 0.5 to 5 mole-% yttrium oxide and having a content of 2to 20 mole % higher than the average, of yttrium oxide in the surfaceregion thereof.
 3. A method for the preparation of a ceramic body whichis partially stabilized with 0.5 to 5 mole-% of yttrium oxide, 5 to 15mole-% of magnesium oxide, calcium oxide and/or cerium oxide or one ormore rare earth dioxides; is 30 to 100% in the tetragonal latticemodification; and has in the surface region a content of yttrium oxide,cerium dioxide, magnesium oxide, calcium oxide or rare earth oxide thatis 2 to 20 mole-% higher than the average content, such that the body iscovered with a thin layer of a more highly stabilized tetragonal latticeform of one predominantly in the cubic lattice form, comprising thesteps ofplacing the surface of an already sintered or only presolidifiedcompact of partially stabilized zirconium dioxide in intimate contactwith yttrium oxide, cerium dioxide, magnesium oxide, calcium oxideand/or other rare earth powder, or a zirconium dioxide powder containingat least 12 mole-% yttrium oxide and/or other stabilizer oxides, andthen annealing said sintered or presolidified compact at 1000° to 1600°C., to form a more highly stabilized tetragonal or mainly cubic surfacelayer of 0.1 to 200 micrometers thickness having a 2 to 20 mole-% highercontent of yttrium oxide, cerium dioxide, magnesium oxide, calcium oxideor rare earth oxide than the average content in the ceramic body.
 4. Themethod of claim 3, wherein the surface contact with yttrium oxide,cerium dioxide, magnesium oxide, calcium oxide or rare earth oxidecomprises placing the ceramic body in a powder bed, spraying or pressinga powder layer onto the ceramic body or treating the ceramic body with asuspension of the powder.
 5. The method of claim 3 wherein the ceramicbody is a body sintered at 1350° to 1550° C. and/or hot-isostaticallypressed and is formed of partially stabilized zirconium dioxide.
 6. Themethod of claim 3 wherein the ceramic body is an unsintered compact ofpartially stabilized zirconium dioxide, and is sintered at 1350° to1550° C.
 7. The method of claim 3 wherein the ceramic body is azirconium dioxide body, partially stabilized with 7 to 11 mole-% ofmagnesium oxide, sintered at temperatures between 1690° and 1800° C.,and thereafter annealed at temperatures between 1350° and 1550° C. incontact with yttrium oxide and/or cerium oxide and/or other rare earthoxide powders for 1 to 5 hours.
 8. The method of claim 3 wherein theceramic body is a sintered compact or green body containing 2 to 4mole-% of yttrium oxide or rare earth oxide as stabilizer.
 9. The methodof claim 4 wherein the ceramic body is sintered at 1350° to 1550° C.and/or hot-isostatically pressed and is formed of partially stabilizedzirconium dioxide.
 10. The method of claim 4 wherein the ceramic body isunsintered compact of partially stabilized zirconium dioxide, sinteredat 1350° to 1550° C.
 11. The method of claim 4 wherein the ceramic bodyis a zirconium dioxide body, partially stabilized with 7 to 11 mole-% ofmagnesium oxide, sintered at temperatures between 1690° and 1800° C.,and thereafter annealed at temperatures between 1350° and 1550° C. incontact with yttrium oxide and/or cerium oxide and/or other rare earthoxide powders for 1 to 5 hours.
 12. The method of claim 11 whereinyttrium oxide, cerium oxide or a combination thereof, is used.
 13. Themethod of claim 4 wherein the ceramic body is a sintered compact orgreen body containing 2 to 4 mole-% of yttrium oxide or rare earth oxideas stabilizer.